Detailed mapping and regional studies indicate that fossiliferous marine formations and nonfossiliferous upland deposits of the Tertiary Chesapeake Group can be integrated into two major sequences that reflect a systematic change in depositional patterns. The lower sequence (Calvert, Choptank, and St. Marys Formations of Miocene age) largely consists of marine-shelf deposits that collectively thicken toward the center of the Salisbury embayment. The upper sequence (Eastover and Yorktown Formations of late Miocene and early to middle Pliocene age) includes complex assemblages of marine-shelf, marginal marine, and nonmarine deposits. A major erosional unconformity and overlying basal deposits of the Eastover Formation are the first evidence of depositional shift southward into the Albemarle embayment.
The Eastover and Yorktown Formations have been recognized and mapped as fossiliferous shelf sediments. Recent fieldwork has shown the two formations to be much more extensive and lithologically variable from the Fall Line to the Chesapeake Bay. Each formation comprises a typical transgressive-regressive sequence. Landward, the base of the
Yorktown is a mappable unconformity truncating the Eastover; seaward, in the lower James River area, marine-shelf deposits of both units converge and are distinguishable only on the basis of their faunas. These units thicken southward toward the Albemarle embayment. Across the same region, the distribution of upper Pliocene and lower Pleistocene transgressive and regressive deposits records a continued trend of tilting toward the southeast.